There are so too options out there and I don't know which way to take. My biggest concerns are damaging plastic parts (will citrus degreaser be too strong for plastic parts?), spending to much money having cheaper choices, and environmental issues (specially the disposing of the stuff).

Any word of advice? Can I use generic degreaser worry-free? Thanks for taking the time of reading this.

First of all, thanks for looking at the rear derailleur cleaning video. A couple of days ago, I posted three videos on how to clean your chain that go into detail about the different kinds of degreaser you're asking about. I think they should answer all of your questions....

Speed Clean dries instantly, so there's really nothing to dispose of. As for the other degreasers, if you look at the very end of the Howcast video, there's a tip on how to dispose of dirty degreaser. It's the very last tip, marked by a green dot at the end of the line below the video. The Howcast video also has a guide that you can print out for easier reading and portability.

As for cost, I tend to go in the other direction: instead of using household products on my bike, I dilute Multi with water and use it for a lot of household cleaning tasks, especially the really nasty ones....
Have fun taking care of your bike.